Problems of correlating ischemic lesions of the brain with the cerebral vessel involved originate in the axial scanning direction. It is almost impossible to line out precisely the borders of the cerebral lobes--far less are we able to describe exactly the areas supplied by the cortical branches of the anterior or middle cerebral artery. Using a method based on detailed perception of the base of the skull and physiological calculations, it is possible to reconstruct the individual baseline of the scan and to project the outlines of the slices upon the lateral view of the angiogram. Thereby a reliable topographical association between CT findings and angiogram is given.